Coaxial transmission lines carrying radio frequency (RF) energy can be used as antennas provided the center conductor of the coaxial line couples RF energy to an aperture in the outer conductor, termed a slot, with sufficient efficiency to cause emission of a significant proportion of the energy applied to the slotted coax/antenna. A typical slotted coaxial line antenna designed for ultra-high frequency (UHF) broadcast, for example, may have from about four to several dozen slots in line, typically occurring at one-wavelength intervals. Such an antenna may also have more than one radially-disposed set of slots.
In order for the slotted coax to radiate efficiently, the two sides of the slot should have a differential distance to the center conductor. This is commonly realized by affixing a conductive rod parallel to the center conductor near or adjacent to one edge of the slot. The impedance mismatch induced by the rod tends to promote radiation out the slot.
Various modifications of the basic concept of the slotted coax antenna and the tradeoffs associated therewith have been attempted by many practitioners of slotted coax design. For instance, it is known that increasing the length of each slot to a full wavelength can be electrically beneficial—but produces a structure with one or more continuous slots, which compromises the mechanical integrity of the antenna. Enlarging the size of each slot near the ends to form a shape known in the art as a “dog bone” can increase the perimeter length while preserving the capacitance at the center of the slot, improving radiation performance but incurring other drawbacks. Inserting a block of higher-dielectric-constant material, such as polytetrafluoroethylene (PTFE, sold for example under the trade name Teflon®) in the slot can reduce the slot's electrical width, but can promote contamination and arcing during extended use. Increasing antenna outer conductor diameter allows the slots to be shorter, but may increase weight and wind drag.
Accordingly, it is desirable to provide an apparatus and method for a slotted coax antenna that increases overall performance with minimal deleterious effects.